The present invention is directed to hydrogen peroxide decomposer for use in water-cooled nuclear reactors, including boiling water reactors and pressurized water reactors, for the mitigation of corrosion phenomena in such systems.
Steel pressure vessels and piping exposed to high temperature water are prone to corrosion due to oxidation of the various metals therein by oxidizing agents, particularly oxygen, present in the high temperature water. Corrosion of such vessels and piping can lead to a variety of problems, including stress corrosion cracking, crevice corrosion and erosion corrosion, leading to leakage and/or bursting of such vessels and piping.
In nuclear reactors, significant amounts of heat energy is generated by reactor processes occurring in the reactor core. A liquid coolant, typically water, is used to remove heat from the reactor core and facilitate its conversion to a useable form. A reactor vessel is provided to contain the reactor coolant around the reactor core to effect such heat removal. Further, piping is provided to facilitate transport of the coolant to steam generators or turbines, where heat energy is ultimately converted to electricity. The materials used in the construction of nuclear reactor vessels and piping are elected for their ability to withstand rigorous loading, environmental and radiation conditions. Such materials include carbon steel, low alloy steel, stainless steel and nickel-based, cobalt-based and zirconium-based alloys.
Despite careful material selection, corrosion and, particularly, intergranular stress corrosion cracking (or, simply, stress corrosion cracking (SCC)), is a problem in steel pressure vessels and piping used in nuclear reactors. SCC, as used herein, refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. Unfortunately, the nuclear reactor environment is conducive to both tensile stressing and corrosion.
Nuclear reactor pressure vessels and piping are subject to a variety of stresses. Some are attributable to the high operating pressure required to maintain high temperature water in a liquid state. Stresses also arise due to differences in thermal expansion of the materials of construction. Other sources include residual stresses from welding, cold working, and other metal treatments.
Nuclear reactors are also susceptible to SCC because of the water chemistry environment of its process systems, which is favourably disposed to corrosion. In this respect, the presence of oxidizing agents, such as oxygen, hydrogen peroxide, and various short-lived radicals, which arise from the radiolytic decomposition of high temperature water in boiling water reactors, contribute to SCC.
Hydrogen peroxide is particularly unstable as it has the ability to act as both an oxidizing agent and a reducing agent. Hydrogen peroxide can act as an oxidizing agent, leading to the formation of water according to the following reaction:
H2O2+2H++2exe2x88x92xe2x86x922H2O
As a reducing agent, hydrogen peroxide is oxidized to oxygen according to the following reaction:
H2O2xe2x86x92O2+2H++2exe2x88x92
Because of its ability to act as both an oxidizing agent and a reducing agent, hydrogen peroxide is highly unstable and will spontaneously decompose into water and oxygen according to the following reaction:
2H2O2xe2x86x922H2O+O2
This will happen if aqueous hydrogen peroxide contacts a metallic surface whose electrode potential lies within this region of instability, which is typically the case in the BWR environment.
Stress corrosion cracking is of great concern in boiling water reactors (BWR""s) which utilize light water as a means of cooling nuclear reactor cores and extracting heat energy produced by such reactor cores. Stress corrosion cracking causes leakage or bursting of such vessels or piping resulting in the loss of coolant in the reactor core. This compromises the reactor process control, which could have dire consequences.
To mitigate stress corrosion cracking phenomenon in BWR""s, it is desirable to reduce the electrochemical corrosion of metal components that are exposed to aqueous fluids. ECP is a measure of the thermodynamic tendency for corrosion to occur, and is a fundamental parameter in determining rates of stress corrosion cracking. ECP has been clearly shown to be a primary variable in controlling the susceptibility of metal components to stress corrosion cracking in BWRs. FIG. 1 shows the observed and predicted crack growth rate as a function of ECP for furnace sensitized Type 304 stainless steel at 27.5 MPa in 288xc2x0 C. water over the range of solution conductivities from 0.1 to 0.3 xcexcS/cm.
For type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni, and 2% Mn), it is known that if the ECP of such steel exposed to high temperature water at about 288xc2x0 C. can be reduced to values below xe2x88x92230 mV (Standard Hydrogen Electrodexe2x80x94SHE) (hereinafter the xe2x80x9ccritical corrosion potentialxe2x80x9d), the stress corrosion cracking problem of such steel can be greatly reduced. The same generally applies for other types of steels.
A well-known method to reduce the ECP to less than xe2x88x92230mVSHE and thereby mitigate SCC of steel pressure vessels and piping in nuclear reactors, is to inject hydrogen gas to the recirculating reactor feedwater. The injected hydrogen gas reduces oxidizing species in the water, such as dissolved oxygen. This has the very desirable benefit of reducing the corrosion potential of the steel vessel or piping carrying such high temperature water.
As illustrated in FIG. 2, ECP of 304SS in 288xc2x0 C. water increases more rapidly with continued addition of hydrogen peroxide when compared to the ECP values measured at the same levels of oxygen concentration. Further, even with the use of hydrogen gas injection, SCC in BWRs continues to occur at unacceptable rates when hydrogen peroxide is present. This is illustrated in FIG. 3, where stress corrosion cracking is shown to occur in BWRs, even with the addition of hydrogen gas, when 20-30 ppb of hydrogen peroxide is present. This information suggests that the presence of hydrogen peroxide in reactor systems is a significant contributor to stress corrosion cracking of metal components. Moreover, the present practice of injecting hydrogen gas into the process liquid does not appear to completely assist in the decomposition of hydrogen peroxide and therefore does not bring about the concomitant reduction in ECP that is expected.
In one broad aspect, the present invention provides a corrosion resistant alloy having a surface exposed to aqueous liquid consisting of oxidizing species, including hydrogen peroxide, that increase the ECP of the alloy. The surface of the alloy is coated with a coating comprised of Mn, Mo, Zn, Cu, Cd, oxides thereof, or chemical compounds thereof. These metals and their compounds assist in causing the decomposition of hydrogen peroxide, thereby reducing the ECP of the alloy. These metals and their compounds can be present as a pre-existing coating on the alloy, or may be deposited in-situ into the aqueous liquid for subsequent deposition on the surface of the alloy after injection.
According to another broad aspect of the present invention there is provided a corrosion resistant alloy cooling tube in a water-cooled nuclear reactor having a surface exposed to an aqueous cooling medium containing hydrogen peroxide, the surface being coated with a coating comprising matter selected from the group consisting of manganese, molybdenum, zinc, copper, cadmium, oxides thereof, chemical compounds thereof and mixtures thereof, for causing decomposition of the hydrogen peroxide.
According to another aspect of the present invention there is provided a water-cooled nuclear reactor comprising metal piping, such metal piping having a surface exposed to an aqueous liquid containing hydrogen peroxide, the surface being coated with a coating comprising matter selected from the group consisting of manganese, molybdenum, zinc, copper, cadmium, oxides thereof, chemical compounds thereof and mixtures thereof, for causing decomposition of the hydrogen peroxide.
According to another aspect of the present invention there is provided a method for lowering the electrochemical corrosion potential of a metal alloy, for use in a cooling tube in a water-cooled nuclear reactor, having a surface exposed to an aqueous liquid containing hydrogen peroxide, comprising the step of coating the surface with matter selected from the group consisting manganese, molybdenum, zinc, copper, cadmium, oxides thereof, chemical compounds thereof and mixtures thereof, for causing decomposition of the hydrogen peroxide.
In a further aspect of the present invention, there is provided a method of lowering the electrochemical corrosion potential of metal alloy cooling tubes in a water-cooled nuclear reactor, the tubes having surfaces exposed to an aqueous liquid containing hydrogen peroxide, comprising the step of injecting matter into said water, said matter selected from the group consisting of manganese, molybdenum, zinc, copper, cadmium, oxides thereof, chemical compounds thereof and mixtures thereof, for causing decomposition of the hydrogen peroxide.